With the advent of low-cost microprocessors, high speed and high resolution pen plotters have become increasingly sophisticated. Most plotters, such as the Hewlett-Packard Co. model 7470A plotter, require that the operator manually load and orient each sheet of paper prior to plotting. This requirement drastically decreases plotter throughput and also increases plotting costs because of the need for operator supervision. A number of plotters, such as the Hewlett-Packard Co. model 7586B plotter, allow roll feeding using a continuous paper roll from which individual sheets are cut to size after plotting is completed. Although roll feeding increases speed and reduces cost, it requires the use of special roll paper which is often disadvantageous in addition to being costly.
In accordance with the illustrated preferred embodiment of the present invention, a microprocessor controlled plotter incorporates an automatic sheet feeder which provides feeding and alignment of individual sheets of paper. The plotter is capable of high speed plotting without operator supervision and allows for the use of low cost and commonly available sheet paper. By the use of stepper motors and linear and angle encoders, paper motion can be monitored with a high degree of accuracy and highly accurate positioning and alignment of the paper can be obtained. Although commonly available photocopiers use various techniques for sheet feeding of copier paper, none of the photocopiers achieves, or requires, the high degree of positioning and alignment accuracy that is achieved by the present invention.
Paper is received from a paper tray by the use of the forward buckling feed technique which is commonly used in low cost photocopiers such as the Canon, Inc., model PC-20 copier. In this pre-feed step, the paper is fed through a deflection door and the leading edge is set between a pair of grit wheels and a pair of pinch rollers. A pen carriage includes a probe which is used to detect paper presence at the platen and which is also used to open and close the deflection door. Once a pre-feed step is successfully completed and paper is detected at the platen, the door is lightly closed and the grit wheels pull the sheet of paper onto the platen until the trailing edge is detected. The paper edge is aligned with an edge perpendicular to pen carriage motion by releasing one grit wheel and forward and reverse feeding the paper a number of times. Rotational forces about the fixed grit wheel cause the paper edge to firmly align against the perpendicular edge.
With the use of high accuracy stepper motors and encoders, the length and width of the aligned paper is determined to a high degree of accuracy and this information may be used by the microprocessor to control subsequent plotting. When plotting is completed, the grit wheels are used to eject the sheet of paper and a new sheet may be fed and aligned on the platen.